I do not feel willing to bring up the scary, scary perhaps to her, diagnosis label of 'Borderline' and 'Disorder'.
You are very wise. This would be counterproductive, and would likely blow up.
I am not a sibling, but the daughter of a pwBPD.
In my experience, and the experience of many people in the books I have read, the "predicament" isn't "how to help fix" the pwbpd.
Accepting that is key.
Instead, we have to work on ourselves, and how to navigate this complex personality while taking care of ourselves.
It's a real shift in thinking.
Many people here have siblings with BPD, so I think you will hear a number of helpful perspectives.
Welcome to our forum!